Method and apparatus for testing frequency-dependent electrical circuits

ABSTRACT

Electrical filter circuits are tested by connecting test apparatus to the filter circuit inputs, without the need to connect to the filter outputs or to disconnect the outputs from a load. A signal generator of known source resistance applies a.c. signals successively over a range of frequencies to the filter circuit inputs, and a voltmeter monitors the voltage across the filter inputs. Different types of filter circuits have different characteristic shapes for the voltage/frequency curve, and processing is applied to the measured results by a computer to determine characteristics of the filter circuit, such as values of the individual sub-components of the filter circuit. An interactive process is applied to successively improve the accuracy of the component value determinations over a range of frequencies. Using the techniques described enables the insertion loss of the filter also to be readily calculated by the computer.

This is a division of Ser. No. 09/952,599, filed Sep. 12, 2001, now U.S.Pat. No. 6,411,101, which is a division of Ser. No. 09/788,009, filedFeb. 16, 2001, now U.S. Pat. No. 6,331 779, which is a division of Ser.No. 09/616,543, filed Jul. 14, 2000, now U.S. Pat. No. 6,252,411, whichis a division of Ser. No. 09/026,789, filed Feb. 20, 1998, now U.S. Pat.No. 6,121,778.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus for testingfrequency-dependent electrical circuits or circuit elements,particularly for example electrical filter circuits or circuit elements.

Electrical filter circuits often need to be tested, for instance for thepurpose of verifying whether they are serviceable or not, i.e. to checktheir operational integrity, or for the purpose of assessing theirelectrical characteristics, or both. The usual method of testing suchcircuits is to apply a.c. (alternating current) input signals ofpredetermined voltage, e.g. of measured and/or fixed standard voltage,from a low-impedance source, over a range of frequencies, to the inputterminals of a filter circuit under test, and to measure thecorresponding voltage delivered at the output terminals.

Most methods for determining the frequency response of an electricalcircuit rely on measurement of both magnitude and phase information atthe inputs and outputs of the circuit in order to derive the frequencyand phase response of the circuit. Once input and output measurementshave been made it has been possible to estimate the insertion loss ofthe filter, i.e. ratio of output to input voltages under the testconditions, usually expressed in decibels, and generally as a functionof frequency. The insertion loss can give desired information about theintegrity or otherwise, and the characteristics, of the filter undertest.

However, increasingly there are situations in which it would bedesirable to test frequency-dependent electrical circuits, for examplefilters, but in which the usual method of testing indicated above is notfeasible. In modern electronic systems, especially where packagingdensities are high, electrical filters are often incorporated indevices. In aircraft for example, the increasing number of pilot andnavigator aids, provided by electrical equipment, means that theintegration of equipment is advanced, and there is little availabilityfor test points within equipment. Furthermore, once the circuits havebeen incorporated into a larger piece of equipment it may beadvantageous to test the circuit for failure. Situations where it is notpossible to have access both to input and to output connections, aswould be required in order to carry out the traditional test methods areincreasingly commonplace. Often the output side is inaccessible.

The present inventor's International Patent Application No. WO95/04935discloses a method and apparatus for testing filters and the like inwhich a.c. input test signals are applied to the input of the filterunder test, and to measurements are made of the voltage across the inputterminals and/or the current through the input terminals, and theintegrity of the filter is determined by identifying a characteristic ofthe resultant measurements. The measurements can simply be compared withthose obtained from a sample filter which is known to be good.Alternatively and preferably the measurements obtained are processed todetect expected characteristics of the measurements. This gives anindication as to whether the filter comes up to specification or not.

That system proves to be very effective, but can only identify a faultyunit. We have appreciated that there would be advantages in being ableto determine why the unit has failed. In particular, it would beadvantageous to determine which of the subcomponents of the filter undertest are giving rise to the problem. This would make it possible torepair the unit, and/or would provide the manufacturers with informationenabling them to improve the quality of their production by removingfrequently appearing faults.

Thus, we have appreciated that it may be desirable to be able todetermine the values of selected subcomponents of the filter under test.It may also be desirable to be able to determine the values ofcharacteristics of the filter such as the insertion loss of the filter.

SUMMARY OF THE INVENTION

The invention in its various aspects is defined in the independentclaims below, to which reference should now be made. Advantageousfeatures are set forth in the appendant claims.

Preferred embodiments of the invention are described below withreference to the drawings. In these embodiments of the inventionelectrical filter circuits are tested by connecting to the filter inputswithout the need to connect to the filter outputs or to disconnect theoutputs from a load. A signal generator of known source resistanceapplies a.c. signals successively over a range of frequencies to thefilter inputs, and a voltmeter monitors the voltage across the filterinputs. Different types of filter have different characteristic shapesfor the voltage/frequency curve, and processing is applied to themeasured results in a computer to determine the location of inflectionsin the curve and other characteristics of the curve. Methods aredisclosed for determining the values of the individual sub-components ofthe filter. Where the filter is an L-C filter, an interactive process isapplied to successively improve the accuracy of the component valuedeterminations. Using the techniques described enables the insertionloss of the filter also to be readily calculated by the computer.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described in moredetail, by way of example with reference to the drawings, in which:

FIG. 1 is a schematic diagram of an arrangement of a computer-controlledtest apparatus for testing frequency-dependent electrical circuits orcircuit elements, particularly for example electrical filter circuits orcircuit elements, according to an embodiment of the invention;

FIG. 2A shows one embodiment of test set-up required to test a parallelcapacitive, or C-section, low-pass filter;

FIG. 2B shows the frequency response obtained when serviceable andunserviceable examples of parallel capacitor filter circuits or circuitelements are tested using the apparatus of FIG. 2A;

FIG. 3A shows the test set-up for a T-section filter;

FIG. 3B shows the test set-up for an L-section filter (which is aspecial case of the T-section filter with L₂=0);

FIG. 3C shows the frequency response characteristic of serviceable andunservice able examples of low-pass T-section or L-section filtercircuits or circuit elements when tested using the apparatus of FIG. 1;

FIG. 4A shows the test set-up required for testing a pi-section filter;

FIG. 4B shows the frequency response characteristic of serviceable andunserviceable examples of low-pass pi-section filter circuits or circuitelements are tested;

FIG. 5 shows a flowchart for testing a pi-section low-pass filter usingthe method of the invention; and

FIG. 6 shows a flowchart for calculating the total capacitance of api-section filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

General Description

A general description of apparatus embodying the invention and itsmethod of operation will first be given, and then the preferredembodiments will be described more specifically in more detail as totheir construction and operation.

One particularly desirable method of obtaining the required measurementis to apply each of a series of a.c. input signals from a signalgenerator, over a predetermined frequency range, successively to acircuit or circuit element under test. The signal generator forms partof a signal source which is of standard or calibrated or otherwisedeterminate voltage (at open-circuit) and which also has a known andfixed source impedance (preferably a purely resistive impedance)associated with it. The resulting voltage developed, at each of theseries of frequencies, between the terminals or connections made to thecircuit or circuit element under test is then measured. Alternatively,the open circuit voltage could be measured using the voltmeter requiredfor measuring the test voltage by switching the filter out of the testcircuit.

The source impedance should be effectively constant over the frequencyrange used for the testing.

For example, a suitable signal source can comprise a variable frequencysignal generator of low output impedance, of standard output voltage (orprovided with means for measuring or standardising its output voltage),and a fixed impedance, e.g. a resistor, in series with its signaloutput. A suitable measurement device can comprise a high-impedancedigital voltmeter connected across the connections made to the circuitor circuit element under test.

In practice the source impedance, usually pure resistance as far as thiscan be realised in practice, has a value chosen to make an appreciabledifference between the resulting voltage at the connections made to thecircuit or circuit element under test, on the one hand, and theopen-circuit voltage of the signal source on the other hand. For example(but without limitation) a 50-ohm source resistance can be used forfilters which have input impedances, at one or more of their cornerfrequencies or characteristic frequencies, in the range of a few ohms(e.g. 1-10 ohm) to a few hundred ohms (e.g. 100 ohm to 1 kohm).

The a.c. input signals are chosen so that they cover a frequency rangewhich ranges well to each side of each corner or characteristicfrequency of the selected type of circuit or circuit element which isunder test, e.g. arrange that includes pass and stop bands. In the caseof a single parallel-capacitative filter, or other low-pass filter, theneeded range extends well both below and above the corner-frequency ofthe configuration which corresponds to the time constant of thecapacitor in conjunction with the associated source and/or loadresistance.

The value of the voltages used is chosen so as to produce minimum activeeffect in the circuits connected to the output of the filter beingtested. These circuits may include active components such astransistors, and thus the voltage applied to the filter is less than 0.7volts. Preferably the voltage is in the range 0.2 to 0.5 volts, and mayconveniently be nominally set at 0.3 volts.

In practice, a useful test method and apparatus catering for a widevariety of important types of filters can utilise a signal generatorwith a range from about 10 kHz to about 50 MHZ, which can sweep thedesired frequency range in suitably chosen linear or logarithmic stepsto demonstrate the characteristic under test. Use of a lower frequencyat the bottom of the range may be desirable if filters incorporatinglarge capacitance values are to be tested, but the indicated range hasbeen found suitable for most filters encountered in practice.

The data-processing arrangement and identification steps of the methodand apparatus embodying the invention can be arranged or performed inany of a number of ways. Examples of such method and apparatus for usewith L-C (inductance-capacitance) filters are described below.

Embodiments of the invention can take the form of a computer-controlledtest rig comprising a digital computer and a variable-frequency a.c.signal generator, of standard or calibratable output voltage, underprogram control by the digital computer, and arranged to feed its a.c.signal through a source resistance to a single pair of connections madeto a circuit or circuit element to be tested. A digital voltmeter isarranged to measure the voltage developed across the single pair ofconnections made to the circuit or circuit element to be tested and todeliver data representing the results of such measurement as input datato the digital computer so as to cooperate with the program control. Thecomputer is arranged under program control to cause a stepwisesuccession of a.c. signals, of a stepwise succession of frequencies, tobe applied to the circuit or circuit element of selected type to betested, and for the data delivered by the digital voltmeter to beprocessed so as to provide for the user of the test rig an indicationrelated to the integrity or performance of the circuit or circuitelement which is under test.

The diagnostic features of the test data show a dependence upon the typeof circuit or circuit element under test, and accordingly it will oftenbe necessary to carry out a different form of comparison or otherdata-processing for each of several such types. Several representativetypes of circuit or circuit element, and the characteristic forms oftest data given by serviceable and unserviceable examples of each, aredescribed below.

Preferred embodiments of the invention are described in further detailbelow, but without limitation on the scope of the invention, and withreference to the accompanying drawings.

General Apparatus for Testing Filters

Referring now to the drawings, FIG. 1 shows the arrangement of testapparatus controlled by a central processor unit (CPU) 1 in accordancewith one embodiment of the invention. CPU 1 may conveniently be anindustry-standard microcomputer such as a personal computer based on a80386, 80486 or Pentium (Trade Mark-s) processor and equipped withhardware expansion slots and/or i/o (input/output) ports such as RS-232ports. Either by hardware expansion cards or by cable links via i/oports, CPU 1 is functionally linked to control a programmablevariable-frequency a.c. signal generator 2 and to receive data from aprogrammable digital voltmeter 3, preferably under control ofmenu-driven software arranged to control the computer to carry out thefunctions described herein.

The connections to the signal generator 2 from the CPU 1, and from thevoltmeter 3 to the CPU 1, may be made via an interface (not shown) ifnecessary. Alternatively an interface sub-routine may be used to allowthe CPU to control the signal generator and voltmeter. The signalgenerator is connected to the two input terminals of the filter 4 to betested, preferably with a source resistor R_(S) arranged in series withthe signal generator to provide an accurately known and stable sourceresistance. Alternatively the signal generator may be provided with asuitable internal resistance as standard which may be used allowing theseparate external resistor R_(s) to be omitted from the test circuit.

The voltmeter is also connected to the two inputs of a filter 4 undertest. The filter (shown as a functional block) has two input terminalsIN and has its output terminals connected to an output or load 5. Theload 5 has a load impedance Z_(L), assumed to comprise a resistor R_(L)and capacitor C_(L) in parallel, shown as connected across the outputsof the filter.

If one connector serves several filters, each of these filters may betested in turn using different pins of the connector. A mechanism forswitching the signals between different pin connections enables all thefilters to be tested without having to reconnect the test equipment.Preferably routing relays, controlled by the CPU, are used.

In a practical example, the filter may be the filter in a combinedfilter-connector, that is a connector which incorporates a filtercircuit or circuits.

The signal generator 2 can be an industry-standard unit chosen (e.g.from the Hewlett-Packard range) to give an output level in this exampleof 0.5V RMS (±5%) over the range 10 kHz to 50 MHz, programmable so thatthe signal frequency is also ±5% relative to nominal programmedfrequency. The generator is chosen to give an output which is sinusoidalwith good purity indicated by low total harmonic distortion preferablynot exceeding −40 dB (decibels), and good linearity over a loadimpedance range from about 5 ohm to about 100 kohm. The generator is ofa type able to drive any impedance value from open-circuit (O/C) toshort-circuit (S/C), and whether resistive or reactive, without damage.In the preferred embodiment, the source impedance of the generator isarranged, using either the series, or internal, resistor R_(S), to beeffectively 50 ohm (as purely resistive as can be realised).

The programmable digital voltmeter 3 is an industry-standard unit chosen(e.g. from the Hewlett-Packard range) to give RMS (root mean square)voltage detection and measurement over the frequency range 10 kHz to 50MHz, over a range of levels from 2 mV RMS to 0.5V RMS, with an accuracypreferably from about 1% of measured voltage at maximum level which maygradually increase to 10% of measured voltage at minimum level. A highvoltmeter input impedance is desired, preferably at least about 100 kohmshunted by no more than 10 pF capacitance over the usable frequencyrange. It is preferred that the measurement accuracy is maintained whenabout 100 different frequency settings are programmed within a period ofabout 10 seconds.

The arrangements described in this embodiment can is conveniently beused for testing filters with capacitive component values up to about0.5 microfarads (μF). If larger capacitance values are likely to beencountered, a lower bottom frequency may be used and the signalgenerator and voltmeter chosen accordingly.

In FIG. 1 the generator 2 and voltmeter 3 are diagrammatically shownconnected to the filter 4 under test. Filter 4 may have any loadimpedance Z_(L) connected across its outputs, as described above, or thefilter may be unloaded and Z_(L) may be absent. In many practicalsituations, the output load may be assumed to consist of a capacitor andload resistor arranged in parallel, as shown. This assumption isexploited by the method for determining the filter component values. Theconnections of a filter 4 under test to both the signal generator 2 andvoltmeter 3 are made via matched coaxial cables 6,7,8,9 connected asillustrated to the two input terminals of filter 4, via a conventionalfour-terminal measurement connector configuration (not shown). Inpractice cables 6 and 9 are constituted by one coaxial cable and cables7 and 8 by another.

It may be convenient to make these connections switchable, e.g. by abank of programmable relays, to any one of a number, e.g. conveniently120 in one example, of plug/socket connectors suitable for receivingfilters to be tested. Such relays can preferably be subject to operationunder software control from CPU 1 in manner known per se.

Especially where such routing relays are used, it can be helpful to takenull-signal measurements from voltmeter 3 when all routing relays areleft open, and to apply any such null-signal values to compensate themeasurements taken when a relay is closed and a circuit or circuitelement is under test.

A problem of equalisation tends to occur in connection with thetransmission of high-frequency measurement signals along such links.Especially where an example of the system is optimised for cables,filters and/or other components having characteristic impedance of e.g.50 ohm, the measurement conditions can be controlled or compensated forinequalities in frequency response by carrying out a calibrationfrequency sweep of the system, (e.g. separately for all relay positionsif a bank-switched system is in use) using a standard signal generatorlevel and a pure resistance of e.g. 50 ohm, in the place normallyoccupied by the (or each) filter to be tested, and storing calibrationdata or compensation data corresponding to the resultant measurements.Any departure from a ‘flat’ frequency response can then be applied as afactor when data-processing the measurement data obtained under testconditions, to compensate for any inequality of frequency response ofthe measurement system.

In the preferred embodiment CPU 1 is connected via an output port 10 toa control input of the signal generator 2. The CPU is provided with thenecessary interface technique, either an interface sub-routine or acompatible signal generator, to enable the CPU to control the frequencyof the output (test) signal applied to the filter 4. Alternatively, thesignal generator may be provided by an add-on board within a standardcomputer, in which case the inputs to the filter 4 are connecteddirectly from the relevant output of the computer.

The digital voltmeter 3 is connected to the input 11 of the CPU,allowing the CPU to read the RMS voltage measured by the voltmeter.Alternatively, exception processing and timers, provided within thecomputer, may be used to provide RMS detection with suitable software,in which case the filter inputs are connected to the input of thecomputer via a buffer if necessary.

The CPU 1 is provided with Read Only Memory (ROM) and programmablememory (not shown). The ROM may contain the operating instructions tothe test procedure including different algorithms encompassing all thedifferent types of filter to be supported. Alternatively, the algorithmsmay be stored on a write protected floppy disk, or magnetic tape, andthe CPU provided with a suitable disk drive. It is now preferred,however, that the program is held in a PCMCIA card and the CPU providedwith an appropriate card reader. The test results can also be held onthe card.

Test Procedure

On initialisation of the test procedure, the user is required to input,for example via a keypad (not shown) associated with the CPU, datacommensurate with the type of filter to be tested. This data may be themodel number of the filter given in the equipment instruction manual orindicated on the equipment, or alternatively the user may select thefilter type from a menu provided on a display (not shown) alsoassociated with the CPU. The CPU then controls which one, or series, ofa number of test algorithms, or test sub-routines, are executed inaccordance with the filter type.

Regardless of filter type, the CPU initiates a procedure which causesthe signal generator to apply a known, and incrementible, frequencysinusoid to the filter. In the embodiments described in detail below, avoltage measurement from the unloaded signal generator is required.Preferably, the voltmeter 3 has been chosen to give a known open-circuitvoltage across the test frequency range. If the open circuit voltage ofthe signal generator varies with frequency, the connections to thefilter from the signal generator and voltmeter may be switched to allowthe open circuit voltage to be measured.

If routing relays are used, the CPU then causes the required relays toclose, connecting the selected filter into the test circuit, and thetest voltage, at a first frequency f₁ is measured by the voltmeter 3,read by the CPU and stored in programmable memory. The test voltage,frequency of the test signal applied, and open-circuit voltage ifnecessary, are stored as associated data forming one entry in a dataset, or table. The processor then sends a signal to the signal generatorto increase the test signal frequency.

At this stage the test may proceed in one of two ways. The processor maycommence data processing according to the specified filter typeprocessing on the data as it becomes available, or a complete frequencysweep can be implemented and all the data stored for subsequentprocessing. In either case the processing principle is the same.

The data is processed to determine whether the voltages measuredindicate that the correct frequency response has been observed. If thecorrect frequency response has indeed been measured, then further dataprocessing, to determine the filter component values and the insertionloss of the circuit, takes place. If the frequency response is found todiffer from the correct frequency response, the filter has failed andsome limited inferences regarding the type of failure may be providedfor the user by a display and/or printout.

The results of the test may be displayed in a number of ways. A visualoutput display of the frequency response may be provided in the case offilter operational failure. In addition, or alternatively, the displaymay indicate the result by displaying a text message. If the filter isoperational, then the component values and insertion loss will bedisplayed in numerical form on the display, and/or by a print-out.

The type of data processing depends on the filter under test. In 90-95%of systems employing pre-filtering, or filtering of the input signal,the filters required are low-pass filters in order to remove highfrequency noise. The description below examines the test procedure ofthe four common types of passive low-pass filters, C-section, L-section,T-section, and pi- (i.e. π) section filters. In principle the method andapparatus may be used to test other types of filter, for example,high-pass or band-pass filters. In these cases the software controllingthe data processing would need to be adapted to the specifics of thefilter under test.

In use, the processor is provided with input effectively consisting ofprogram software P to perform the functions described herein, and asetting S set by the operator, to correspond to the type of filter to betested, e.g. one of the types described in connection with FIGS. 2 to 4below. Setting S then determines which of several variants of the dataprocessing are to be carried out in connection with the test of thefilter 4. The computer 1 provides results R in any convenient form,provided by software in manner known per se.

Low-band-pass L-C filters

FIG. 2B, FIG. 3C and FIG. 4B show the frequency responses of the filtertypes shown in FIG. 2A, FIGS. 3A and 3B, and FIG. 4A respectively whenthe above described apparatus is used to test each type of filter. Theforms of the frequency response for these types of filter are wellknown. The frequency response of the filter is verified by the systemand used, as described below, to ascertain the filter component valuesand the insertion loss of the filter, under the assumption that the loadmay be modelled as a parallel combination of capacitance and resistance.In practice, this assumption is often valid.

As noted above, the user must input information to the processor whichwill enable the processor to ascertain the type of filter, for example afilter model number displayed on the equipment may be entered which theprocessor can identify from a look-up table.

The system uses the recognition of the shape of the frequency responseto determine whether the filter is operational. If the operation of thefilter is verified, the component values and finally the insertion lossof the filter are estimated. If the filter has failed, the processor candetermine from the frequency response that the filter has failed, andprovide some information regarding the type of failure, that is, as towhich components in the filter have become open-circuit orshort-circuit.

The raw RMS voltage readings are first converted by calculating theratio of the test voltage to the open circuit voltage, and expressingthe result in decibels. The converted voltage reading is associated withthe logarithm of the test frequency, thus forming a data set. The datasets may be used to plot a frequency response on alogarithmic-logarithmic scale of voltage ratio (in decibels) versusfrequency. However, the verification of the frequency response, andsubsequent data processing required to estimate the filter componentsand the insertion loss of the filter, is performed by mathematicalprocessing of the data set by a computer using an iterative method toestimate the filter components. The test apparatus may be used to testcapacitive and LC filters which on their output sides are eitherunloaded, or loaded where the load may be modelled by a resistor andcapacitor arranged in parallel. The overall load impedance is denotedZ_(L). However, the main features of the frequency response, such ascurve 1-1 a in FIG. 2B, defined by the overall shape of the frequencyresponse, are substantially unaffected over a range of load conditions.It is, therefore, generally unnecessary to have prior knowledge of thenature or magnitude of any such load impedance.

If the load comprises active components, the filter should be testedwith the load unpowered. It is an aim of the invention that the test maybe performed in situ. In practice, therefore, the environment shouldpreferably be as noise-free as possible during testing and surroundingelectrical equipment should normally be unpowered.

Characteristic Frequency response of a C-section filter.

FIG. 2A is a circuit diagram of the system when the filter being testedis a C-section or parallel capacitive filter. The capacitance is shownas C₁ in filter 4 in FIG. 2A.

FIG. 2B shows the frequency response obtained when serviceable andunserviceable examples of a parallel capacitive filter, or C-sectionfilter, are tested using the apparatus of FIG. 1. The function:

|20×log₁₀(Z _(L)/(Z _(L) +R _(S)))|

(ordinate) against the logarithm of frequency (abscissa, with highfrequencies to the right in the diagram of FIG. 2B) is plotted to givethe frequency response of the filter, where Z_(L) is the parallelimpedance of C₁, C_(L) and R_(L) (as shown in FIG. 2A) at any frequency,in series with the source resistance R_(S).

Curve 1-1 a of FIG. 2B shows the characteristic frequency response foran operational C-section filter If capacitor C₁ is faulty andopen-circuit, then the response of the filter under test is independentof frequency and the falling portion of the normal characteristic, i.e.curve portion 1 a, is replaced by curve portion 2. If, on the otherhand, the capacitor is faulty and short-circuit, the response is againfrequency independent but the overall resistance of the filter is lower,causing curve 1-1 a to be replaced by curve 3.

In order to determine the serviceability of the filter the dataprocessing is arranged to discriminate between curves of the types 1, 2and 3, and thereby to provide an output which is indicative of the stateof the component under test, whether it is serviceable, open-circuit orshort-circuit.

Testing C-section low-pass filters: Measurement

The data processing software, when set to examine a filter of this type,is arranged to analyse a data set of number-pairs each representing apoint or one of the curves 1, 2 or 3 of FIG. 2B. These number-pairsresult from applying a.c signals of a range of frequencies, at standardvoltage, to the C-section filter under test As noted above, eachnumber-pair consists of a digital numerical representation of one of theseveral test frequency values, paired with a digital representation ofthe corresponding voltage measurement made by voltmeter 3 across theinput of the filter 4 when the signal of that frequency is applied.

With the filter 4 electrically isolated from the signal generator 2, ieswitched out of the circuit for example using routing relays, theprocessor reads the value of the voltmeter 3 using a suitable interfacedetector sub-routine, and this value is stored by the processor as theopen circuit voltage. If the open circuit voltage alters with frequency,either measurements must be taken at each test frequency or themanufacturer's handbook must be consulted in order to obtain opencircuit voltages at the test frequencies. The filter is then connectedinto the circuit using an interface relay sub-routine controlled by theprocessor.

The test voltage across the input terminals of the filter 4 is thenmeasured using the voltmeter 3. The CPU 1 reads the RMS voltage acrossthe input terminals of the filter from the voltmeter and stores theresult in association with the frequency, and if necessary with theopen-circuit signal voltage at that test signal frequency. The CPU 1then sends a control signal, generated in accordance with appropriateinterface generator sub-routines, to the signal generator 2 to increasethe frequency of the output sinusoidal waveform, and the new testvoltage is measured, and the value stored. In practice, the testvoltages at each of a range of test frequencies are usually stored in anarray in ascending frequency order, each voltage being implicitly linkedto a test frequency.

The test may proceed in one of two ways; either a complete frequencysweep is made, or the frequency sweep may be limited by the results ofearlier data processing. If a complete frequency sweep is to be madethen the above measurements are repeated at increasing frequencies andthe entire set of results is obtained prior to any data processing. Inone example the increasing test frequencies may be the logarithmicprogression 31.62 kHz, 100 kHz, 316.2 kHz, 1 MHz, 3.16 MHz, 10 MHz and31.52 MHz. However, limited data may provide enough information todetermine, and verify, the shape of the filter, and it may prove moreefficient to introduce an element of data processing during the testingprocedure. In either case, further measurements, tailored to thespecific type of filter, are required.

Estimation of the cut-off frequency

The basic testing and processing procedure for finding the cut-offfrequency of the filter is the same for C-, L-, T- and pi-sectionlow-pass filters. Subsequent processing, to verify the characteristicfrequency response for L-, T-, and pi-section filters is required priorto estimation of the filter components. The method below assumes anominal datum frequency of the test signal of 10 kHz. In practice, thisdatum frequency is chosen with regard to the signal generator. Frequencysteps are also chosen with regard to the signal generator. It is asimple step to adapt the method to accommodate any test frequencies. Theexample below is illustrated with a limited set of readings determinedat frequencies of 10 kHz, 31.62 kHz, 10 kHz, 316.2 kHz, 1 MHz, 3.162MHz, 10 MHz and 31.62 MHz. In the presently preferred embodiment,however, 131 frequency settings are used at closer spacings over thefrequency band of 10 kHz to 50 MHZ. This allows the data to be smoothed,using known windowing techniques, and improves the detection of theturning points for the pi-section filter using the differencing methoddescribed below. The frequencies are chosen to be substantially equallyspaced on a logarithmic scale.

Assuming the more-limited frequency set is used, once readings of thetest voltages, and if necessary the open circuit voltages, atfrequencies of 10 kHz and 31.62 kHz have been obtained, the ratio of thetest voltage V_(T) to the open circuit voltage V_(OC) each frequency isestimated in dB, ie the ratio 20 log₁₀(V_(T)/V_(OC)) dB is estimated. Ifthe difference between the ratios obtained at 10 khz and at 31.62 kHz isgreater than 3 dB, then it is assumed that the cut-off frequency hasbeen exceeded, and an estimate of the actual cut-off frequency can bemade by interpolation between the measured data sets. If a sufficientlylarge number of steps are used, interpolation may not be necessary. Ifthe difference is less than 3 dB, the cut-off frequency has not beenreached and the processor sends a signal to the signal generator toincrease the frequency of the test signal to say 100 kHz. The new opencircuit and test voltages are measured and stored. The ratio between thetest voltage and the open circuit voltage at 100 kHz is now calculated,and compared with the 10 kHz datum value.

The test frequency is increased in this way in predetermined steps,until the difference between the latest voltage ratio and the 10 kHz (ordatum) voltage ratio is greater than 3 dB. In practice it may be thatnone of the test frequencies chosen precisely coincide with the cut-offfrequency because of the finite step sizes. The CPU, therefore,interpolates between the data points to find the cut-off frequency.Denoting the lowest test frequency at which the voltage ratio is morethan 3 dB less than the datum frequency voltage ratio as Fm, and thevoltage ratio at Fm as dBm, the cut-off frequency, F_(3dB), is found inaccordance with equation (1): $\begin{matrix}{F_{3{dB}} = {{F_{M}\frac{1}{\left( {\frac{1}{y^{2}} - 1} \right)^{\frac{1}{2}}}\quad {where}\quad y} = {{antilog}_{10}\frac{dBm}{20}}}} & \text{[Equ~~(1)]}\end{matrix}$

Failure Modes for C-section filters

If, at any test frequency, the measured test voltage falls below 10 mV(RMS), or the ratio of test voltage to open circuit voltage is estimatedto be greater than 34 dB, then the test is deemed to have failed. Thereare three types of fault which could cause this result: the filter loadresistance is below 1 ohm and is outside the specification of theinstrument, the capacitance of the filter is greater than 17 μF and isoutside the specification of the instrument, or the capacitor has goneshort circuit. In any of these circumstances, the frequency response isnot of the characteristic form. If the frequency response of the filteris not of the expected form, calculation of the filter components andinsertion loss are impossible. However, the user is alerted to thefailure of the unit, allowing it to be replaced.

Limitations of test equipment

Since a signal generator generally has a maximum frequency of 50 MHz,the detection of the capacitance values are limited by the filter loadimpedances. As a general guide, for a maximum frequency of 50 MHz and afilter load impedance greater than 50 ohms, the minimum filtercapacitance that can be detected is 150 pF; for a filter load of 10 ohmsthe minimum filter capacitance that can be detected is 500 pF; and for aload resistance of 1 ohm the minimum filter capacitance that can bedetected is 5.0 nF.

Estimation of the filter components for C-section filters.

In order to estimate the filter components, an estimate of the loadresistance R_(L) must first be made. In the preferred embodiment thesource resistance R_(S) is chosen to be 50 ohms. In practice, the sourceresistance must be known but need not be 50 ohms. For a low-pass filterat a frequency of 10 kHz, any capacitance of the load C_(L) of thefilter C₁ is negligible, and the resistive component R_(L) of the loadmay be related using Ohm's Law to the open circuit voltage, the testvoltage and the source resistance, which are all either known ormeasured. The relationship between the open-circuit voltage of thegenerator V_(OC) and the voltage V_(LOAD) at the filter inputs is givenby: $\begin{matrix}{\frac{V_{LOAD}}{V_{OC}} = {\frac{R_{L}}{R_{S} + R_{L}} \cdot}} & \text{[Equ (2)]}\end{matrix}$

Alternatively, the result can be estimated from the frequency responseusing the voltage ratio in dB at 10 kHz: $\begin{matrix}{{dB} = {20\log_{10}{\frac{R_{L}}{R_{S} + R_{L}} \cdot}}} & \text{[Equ (3)]}\end{matrix}$

By rearrangement: $\begin{matrix}{R_{L} = {{\frac{X\quad R_{S}}{\left( {1 - X} \right)}\quad {where}\quad X} = {{antilog}\frac{{dB}_{10\quad {kHz}}}{20}}}} & \text{[Equ~~(4)]}\end{matrix}$

and R_(L) can be estimated by the processor.

At low frequencies, regardless of the filter type, the resistance R_(e)of the capacitor C₁ is negligible, and the effective resistance R_(P) ofthe filter 4 is related to the source and load resistances R_(S) andR_(L) as follows:$R_{P} = {\frac{R_{L}R_{S}}{\left( {R_{L} + R_{S}} \right)} \cdot}$

The load resistance R_(L) has been calculated, the source resistanceR_(S) is known, and, hence, the effective resistance R_(p) of the filtermay be calculated.

For a C-section filter, the value of the filter capacitance C₁ can becalculated from the cut-off frequency and the effective resistance.

In addition, on curve 1-1 a, consider point B which is exactly 3 dBlower than dB_(10 kHz) (or the lowest measurement frequency). At thispoint: $\begin{matrix}{\left( {C_{1} + C_{L}} \right) = \frac{1}{2\Pi \quad F_{3{dB}}R_{P}}} & \text{[Equ~~(5)]}\end{matrix}$

where:

(C₁+C_(L))=total circuit capacitance C_(T)

F_(3 dB)=the frequency at point B

R^(P)=effective circuit resistance=$R_{P} = \frac{R_{L}R_{S}}{\left( {R_{L} + R_{S}} \right)}$

 and hence the total circuit capacitance can be calculated.

Estimation of the insertion loss for a C-section filter

The insertion loss is frequency dependent and in accordance with anaspect of this invention can be deduced from an estimate of themagnitude of the attenuation A of the filter at any frequency. Bysubstituting the known values, the attenuation may be expressed in termsof a frequency response. The insertion loss, I_(L), may then beestimated. The data processing software, when set to examine a filter ofthis type, is arranged to calculate the insertion loss from the curve inFIG. 2B. The magnitude of attenuation AdB at any frequency f can bededuced from: $\begin{matrix}{A = {20\log_{10}\frac{1}{\left\lbrack \left( {1 + \left( \frac{R_{S}}{R_{L}} \right)^{2} + {\omega^{2}R_{S}^{2}C_{T}^{2}}} \right\rbrack^{\frac{1}{2}} \right.}}} & \text{(Equ~~(6)]}\end{matrix}$

where ω=2.Π.f. The insertion loss I_(L) at any frequency is given by:$\begin{matrix}{I_{L} = {\left\lbrack {A - {20\log_{10}\frac{R_{L}}{R_{S} + R_{L}}}} \right\rbrack {dB}}} & \text{[Equ~~(7)]}\end{matrix}$

where R_(S) is known, and R_(L), C_(T) and A have been calculated fromequations (4), and (5) and (6) respectively. Writing equations (6) and(7) as a single equation gives:$I = {{20\log_{10}\frac{1}{\left\lbrack {\left( {1 + \frac{R_{S}}{R_{L}}} \right)^{2} + {\omega^{2}R_{S}^{2}C_{T}^{2}}} \right\rbrack^{\frac{1}{2}}}} - {20\log_{10}{\left\lfloor \frac{R_{L}}{R_{L} + R_{S}} \right\rfloor \cdot}}}$

It can be useful to arrange that the frequency response is displayedvisually on a plot providing a useful way for the operator to judge theresult of the test procedure.

L- and T-section Filter Test Procedure

FIG. 3A shows the test set-up for a T-section filter. A T-section filterhas two inductors L₁ and L₂ and one capacitor C₁. The inductors areconnected in series between one input terminal and one output terminalof the filter, and the capacitor is connected in parallel between thejunction between the inductors, on the one hand, and the other of eachof the input and output terminals of the filter, on the other. TheL-section filter is a special case of the T-section filter andcorresponds to inductance L₂ having a negligible value and beingreplaced by a short circuit as shown in FIG. 3B.

FIG. 3C shows the frequency response obtained when serviceable andunserviceable examples of L-section and T-section low-pass L-C filtercircuits or circuit elements are tested using the apparatus of FIGS. 3Aor 3B. The voltages measured for a serviceable example of L-, or T-,section filter result in a frequency response of the form of solid curve1 (continued at 1 a and 1 b) shown in FIG. 3C. The pronounced dip in thelog-log plot is characteristic of these types of filters and can be usedto identify that the filter is in serviceable condition, free fromshort-circuit and open-circuit faults.

If the capacitance C₁ is open-circuit, then the is frequency responsefollows curve 1 initially but rises along curve 2 a instead of fallingalong curve 1 a, and the characteristic dip is absent. If the inductanceL₁ is open circuit (i.e. in this connection is of substantially zeroinductance, e.g. because the ferrite bead that should provide it hassplit and fallen off its associated wire), the frequency responseinitially follows curve 1 and 1 a but fails to turn upwards along curve1 b, following instead curve 2 b. If the capacitance C₁ is short circuitthen the response is frequency independent and the characteristicresponse is replaced by curve 3.

By comparing the measured frequency response with the characteristicfrequency response for this type of filter, the operational integrity ofthe filter can be assessed. In practice, the response may not be aspronounced or clean as the characteristic response. The CPU shouldtherefore desirably be able to detect whether the response is analogousto the characteristic response from imperfect data. It follows from theshape of the frequency response that both L- and T-section filters havecut-off frequencies which can be estimated in the same way as for theC-section filter.

Estimation of the cut-off frequency

The test procedure initially follows that of the C-section filter,described above, with open-circuit and test voltages being measured fora range of frequencies. Once the cut-off frequency of the filter hasbeen established, the processor must also establish that the frequencyresponse subsequently rises at higher frequencies in order to verify thecharacteristic frequency response and operational integrity of thefilter.

If the response does not fall by 3 dB or more below the 50 MHzmeasurement, then the capacitance is open circuit and the filter isdefective. In this case, the filter components and insertion loss cannotbe estimated. In practice, this would enable an inoperative filter to bereplaced.

Verification of the frequency response for L- and T-section filters.

In order to establish that the frequency response of the filter iscorrect, the programmable signal generator 2 should then be set toprovide a test signal of the cut-off frequency of the filter. At thecut-off frequency, the test voltage is measured. The processor firstchecks the ratio of test to open-circuit voltage to ensure that it is 3dB below the datum value as expected. If the test fails, the processorrecalculates the cut-off frequency and retests at the new cut-offfrequency until the cut-off frequency is established. On alogarithmic-logarithmic scale, the frequency response of the filter willfall at 20 dB/decade until the effect of the inductors causes thefrequency response to rise. Once the cut-off frequency has beenestablished, the shape of the frequency response can be verified bychecking that the response subsequently rises. If the response rises thedifference in dB between the datum frequency reading and the currentfrequency reading will be less than the difference between the datumfrequency and the previous test frequency reading. Given that theresponse drops at 20 dB/decade, a threshold can be calculated for anyfrequency above the cut-off frequency f_(cutoff) which will determinewhether the frequency response has begun to rise. The example below usestest frequencies of 3.162×f_(cutoff), 10×f_(cutoff), 31.62×f_(cutoff),and 100×f_(cutoff).

The processor sets the programmable generator to the frequency given inthe first row of Table 1 appended to this description, i.e.3.162×f_(cutoff). The processor checks that the voltage in dB at thishigher frequency is less than 10 dB±0.5 DB lower than the value at thedatum frequency, the threshold shown in the second column of Table 1. Ifthe test condition is not met, the programmable generator is set to thenext frequency, i.e. 10×f_(cutoff), measuring the test voltage andconverting it to dB. The new reading is compared with the correspondingthreshold. For so long as the condition is not met, the frequencyresponse has not begun to rise and the shape of the filter isunverified. The frequency must be increased and the next threshold used.If different frequencies are used, the thresholds need to berecalculated using the fact that the fall-off is 20 dB/decade toestimate the new thresholds. The test is continued at higher frequenciesuntil the test condition is met. At this frequency the voltage ratio hasbegun to rise along curve 1 b as shown by the characteristic frequencyresponse in FIG. 3C and the shape of the L- or T-section filter has beenverified.

Another method of determining the minimum frequency is described belowin relation to pi-filters.

If a generator frequency of 50 MHz is reached i.e. the maximumfrequency, and the test condition has not been met, then the filter isfaulty, and inductance L₁ of the T network is open-circuit. On the otherhand, if the operation of the T filter is verified, the filtercomponents can then be calculated. Note that it is not possible usingthis procedure to establish whether inductance L₂ is satisfactory.

Estimation of the filter component values for L- and T-section filters.

In order to estimate the filter component values, the filter loadresistance, R_(L), must first be estimated. For L- and T-sectionfilters, over the pass-band the resistive component of the inductors andcapacitors will be negligible. The load impedance, Z_(L), is assumed tocomprise a parallel combination of resistance R_(L) and capacitanceC_(L). The inductance of the T filter has two components due to the twoinductors L₁ and L₂. The inductors have associated loss resistancesR_(e1), and R_(e2) which are frequency variable. At 10 kHz, theseassociated loss resistances are negligible, but at frequencies ofF_(3 dB) and above the resistances are significant. The value of R_(L)can be calculated as for the C-section filter, e.g. from equation (4).

The total capacitance C_(T) of the circuit is a function of theeffective resistance R_(P) and can be calculated from equation (8):$\begin{matrix}{C_{T} = \frac{1}{2\Pi \quad F_{3{dB}}R_{P}}} & \text{[Equ~~(8)]}\end{matrix}$

where:

C_(T)=C₁+C_(L) $\begin{matrix}{R_{P} = {\frac{R_{S}\left( {R_{L} + R_{e1} + R_{e2}} \right)}{R_{S} + R_{L} + R_{e1} + R_{e2}} \cdot}} & \text{[Equ~~(8)a]}\end{matrix}$

There are too many unknown variables to determine the value of C_(T) atthis stage. In order to proceed the values of R_(e1) and R_(e2) are setto zero, i.e. R_(e1)=R_(e2)=0, in order to estimate an interim value forR_(p). Further information is available from the frequency response byconsidering the minimum in the frequency response, at frequency f_(min).The minimum of the frequency response is estimated using an inflectionalgorithm. The inflection algorithm is described below for thepi-section filter. The minimum in the frequency response is caused bythe resonance of the inductor L₁ and capacitor C₁. At this point L₁ andC₁ are related by equation (9): $\begin{matrix}{L_{1} = {\frac{1}{\left( {2\Pi} \right)^{2}\left( f_{\min} \right)^{2}C_{1}} \cdot}} & \text{[Equ~~(10)]}\end{matrix}$

In general for a T filter, C_(L) will be small compared with C₁, inwhich case C_(T)=C₁ and C_(T) is given by equation (8). Also theinductors L₁ and L₂ take the same value, L₁=L₂, making R_(e1)=R_(e2) inpractice. R_(e1) may be related to the minimum of the frequencyresponse: $\begin{matrix}{R_{e1} = \frac{D\quad R_{S}}{\left( {1 - D} \right)}} & \text{[Equ~~(10)]}\end{matrix}$

where: $\begin{matrix}{D = {{{antilog}_{10}\left( \frac{{dB}\quad {at}\quad f_{\min}}{20} \right)} \cdot}} & \text{[Equ~~(11)]}\end{matrix}$

Given that the above assumptions hold, a value for R_(e1)=R_(e2) can beused to estimate a new improved value for C_(T) in Equation (8) andhence new values for R_(e1) and R_(e2). This iterative loop is repeateduntil successive values of C_(T) are within a predetermined amount suchas 1% of each other, and this is then taken to be the value of componentC_(T). Finally, the values of L₁=L₂ can be calculated from equation (9).

Estimation of the insertion loss for T- and L-section filters.

The insertion loss I_(L) of the T filter can be found by calculating:$I_{L} = {{20{\log_{10}\left\lbrack \frac{Z_{L}Z_{3}}{\begin{matrix}{{Z_{2}R_{S}} + {Z_{2}Z_{1}} + {Z_{2}Z_{3}} + {Z_{L}R_{S}} +} \\{{Z_{L}Z_{1}} + {Z_{L}Z_{3}} + {Z_{3}R_{S}} + {Z_{3}Z_{1}}}\end{matrix}} \right\rbrack}} - {20{\log_{10}\left\lbrack \frac{Z_{L}}{Z_{L} + R_{S}} \right\rbrack}}}$

where:

Z₁=R_(e1)+jωL₁

Z₂=R_(e2)+jωL₂ $Z_{3} = \frac{1}{{j\omega}\quad C_{1}}$

Z_(L)=R_(L)

The insertion loss I_(L) of the L filter is the same but with L₂ setequal to zero. Thus the insertion loss of the L filter can be found bycalculating:$I_{L} = {{20{\log_{10}\left\lbrack \frac{Z_{L}Z_{3}}{\begin{matrix}{{Z_{L}R_{S}} + {Z_{L}Z_{1}} + {Z_{L}Z_{3}} +} \\{{Z_{3}R_{S}} + {Z_{3}Z_{1}}}\end{matrix}} \right\rbrack}} - {20{\log_{10}\left\lbrack \frac{Z_{L}}{Z_{4} + R_{S}} \right\rbrack}}}$

Pi-section filter test procedure

FIG. 4A shows the test set-up for a pi-section low-pass filter. Thepi-section filter 4 has one inductor L₁ and two capacitors C₁ and C₂.Capacitor C₁ is connected across the two input terminals and capacitorC₂ is connected across the two output terminals. Inductor L_(T) couplesone input terminal to one output terminal, the other of each of theinput and output terminals being connected together. The characteristicfrequency response is shown in FIG. 4B. The frequency response obtainedfor serviceable and unserviceable examples of a pi-section low-pass L-Cfilter circuits or circuit elements are shown. The data given by aserviceable example of such a filter creates a frequency response of theform of solid curve 1 starting at 1 a and continued at 1 b, 1 c and 1 din FIG. 4B, with a noticeable S-shape in this log-log plot. Thiscorresponds to an example of the filter in serviceable condition, freefrom short-circuit and open-circuit faults. The types of load normallyapplied to such a filter have little effect on the overall shape of thefrequency response, shifting the whole response horizontally orvertically depending on the reactance of the load.

If remote capacitance C₂ is open circuit, then the response correspondsto a curve made up of portions 1 a, 2 a and 1 d. If the capacitance C 1is open-circuit then the frequency response approaches that of anL-section filter i.e. with a dip and a rise, following curve 1 a, 1 b, 1c, and 2 b. If the inductance, L or L₁, is open circuit, then thefrequency response corresponds to curve portions 1 a, 1 b and 2 c. Ifeither capacitance C₁ or C₂ is short circuit, then the curve is replacedby curve 3. An inductor provided by a ferrite bead threaded around awire does not generally develop a short-circuit fault, though itsinductance may largely disappear if the bead is damaged or missing.

As shown, curve portion 1 c has positive slope on FIG. 4B. Depending onthe component values, however, it may not be such a sharp inflection andmay have a negative slope, which is nevertheless less than the negativeslope of curve portions 1 b and 1 d, which have substantially the sameslope. The system will however detect a less-pronounced inflection ofsuch a type.

The steps in verifying the frequency response, and estimating the filtercomponent values and the insertion loss of the filter, are shown in theflowchart of FIG. 5. This flowchart will be self-explanatory to thoseskilled in the art in conjunction with the above description and averbal description is not therefore repeated here.

Estimation of the cut-off frequency

The characteristic response for the pi-section filter is similar to thatof the L- or T-section filters at low frequencies and differs in thatthe response rises and then falls, being effectively characterised by anS-shape. Using the method described above with reference to theC-section filter, the cut-off frequency of the filter is firstdetermined.

Verification of the frequency response for a pi-section filter.

The process for determining that the frequency response rises is thesame as for the L- and T-section filters. Once the rise of the frequencyresponse has been detected the fall-off of the response must beverified. Denoting the frequency at which the response begins to rise asF_(rise), the signal generator frequency is increased by a factor of3.162, for example if F_(rise)=10×F_(cutoff), then the signal generatorfrequency is set to 31.62×F_(cutoff). At this new frequency, thedifference in dB between the previous value and the current value shouldbe greater than 10 dB, with the current value being more than 10 dBlower than the previous value. If this condition is met, the filter isfunctional. If the new value falls short of being 10 dB higher than theprevious value, then capacitor C₁ is open-circuit.

Once the frequency response has been verified, i.e. the shape of theresponse has been shown mathematically to correspond to thecharacteristic shape of the particular filter, the component values ofthe filter may be estimated.

Estimation of the filter component values for a pi-section filter

The filter load impedance is assumed to comprise a parallel combinationof resistance R_(L) and capacitance C_(L). The inductance of the pifilter comprises an inductor L₁, and associated loss resistance R_(e)which is frequency dependent and which is negligible at 10 kHz, butwhich has a significant value at F_(3dB) and above.

The value of R_(L) is estimated in the same manner as for C-, L- andT-section filters using the 10 khz datum frequency test data andEquation (4). The total capacitance of the circuit is given by:$\begin{matrix}{C_{T} = \frac{1}{2\Pi \quad F_{3d\quad B}R_{p}}} & \left\lbrack {{Equ}\quad (12)} \right\rbrack\end{matrix}$

where: $\begin{matrix}{R_{p} = \frac{R_{S}\left( {R_{L} + R_{e}} \right)}{R_{S} + R_{L} + R_{e}}} & \text{[Equ~~(12a)]} \\{C_{T} = {C_{1} + C_{2} + C_{L}}} & \quad\end{matrix}$

Note that, at this stage of computation, R_(e) at F_(3dB) is unknown anda value of R_(e)=0 is used to obtain an interim value of R_(P).

Estimation of the inflection points for a pi-section filter.

Further information is available in the inflection points of thefrequency response. A curve inflection derivation method is used whichis based on the difference in the voltage ratio between the open-circuitand test voltages in dB at different frequencies. In practice, 131 testfrequencies, equally spaced on a logarithmic scale, are used in themeasurement of the test voltage and subsequent calculation of the dBlevel at the test voltage. The results are placed in an array infrequency order and then smoothed using a window length of 5 samples.The first and last two samples remain unchanged in the array due to thewindow length. Once smoothed, the inflection points of the array must beestimated. For regularly spaced frequency readings, the slope of thecurve is given by the difference in dB between any two of the readings.In practice, a smoothing window length of 7 samples is used, i.e.samples n and (n+7) are used, hence:

dy/dx=(dB[n]−dB[n+7])/7.

A new array is used to store the difference values, with the first andlast three entries being null due to the length of the difference windowchosen. Inflection point C, which as shown in the minimum in the curve,is estimated by determining the lowest frequency at which the differencevalue falls below the average of all the difference values The minimumfrequency of an L- or T-section filter frequency response may also becalculated in this manner. Inflection point D, which as shown in thesubsequent peak, is then estimated as the first entry in the differencearray after point C which is greater than the average value. At point Cdenote the dB level=dB_((fmin)), and frequency=f_(min). At point Ddenote the dB level=dB_((fmax)) and the frequency=f_(max).

Using the fact that the frequency response drops at 20 dB/decade for aC-section filter, the test data may now be extrapolated to estimate apoint F. The value in dB at point D is used to establish a point E oncurve portion 1 d of FIG. 4B which has a value 3 dB lower than point D:

dB_(E)=(dB_((fmax))−3)

where the frequency at this point E is denoted f_(b).

The position of the point F, corresponding to a frequency on curveportion 2 c of FIG. 4B with the same value in dB as point E, has afrequency f_(a), the attenuated frequency, where:

f _(a) =F _(3 dB) ×A  [Equ 13]

where:

A=antilog₁₀{(dB_(10 kHz)−dB_(E))/20}.

The processor then estimates the extrapolated frequency ratio k where:$\begin{matrix}{k = \frac{f_{b}}{f_{a}}} & \left\lbrack {{Equ}\quad (14)} \right\rbrack\end{matrix}$

and calculates the value of the first capacitance C₁ from:$\begin{matrix}{C_{1} = \frac{C_{T}}{k}} & \left\lbrack {{Equ}\quad (15)} \right\rbrack\end{matrix}$

In general, for pi filters, C₁=C₂, that is both internal capacitancesare equal. In this case:

C _(L) =C _(T)−2C ₁  [Equ (16)]

At the first minimum after F_(3 dB), that is at f_(min), the inductanceL is in resonance with (C₂+C_(L)). Hence, the effective inductance ofthe circuit is given by: $\begin{matrix}{L = \frac{1}{\left( {2n} \right){f_{\min}^{2}\left( {C_{2} + C_{L}} \right)}}} & \left\lbrack {{Equ}\quad (17)} \right\rbrack\end{matrix}$

Also at this frequency f_(min) it is possible to derive an equation forR_(e): $\begin{matrix}{R_{e} = \frac{{BR}_{S}}{\left( \left\lbrack {\left( {1 - B} \right)^{2} + {B^{2}\omega^{2}C_{1}^{2}R_{S}^{2}}} \right\rbrack \right)^{\frac{1}{2}}}} & \left\lbrack {{Equ}\quad (18)} \right\rbrack\end{matrix}$

where:

ω=2Π f_(min) $\begin{matrix}{B = {{antilog}_{10}\left\lbrack {{{dB}'}s\quad {at}\quad \frac{f_{(\min)}}{20}} \right\rbrack}} & \left\lbrack {{Equ}\quad (19)} \right\rbrack\end{matrix}$

The processor now replaces the initial value for R_(e) in equation (12a)to give a new value for C_(T) in equation (12) and hence a new value forC₁ in equation (15) and a new value for R_(e) in equation (18). Thisinteractive loop is repeated until successive values of C_(T) are withina predetermined amount such as 1% of each other. FIG. 6 shows aflowchart corresponding to the estimation of C_(T) as described above.As with FIG. 5, a verbal description of this Figure is unnecessary.

Estimation of the insertion loss of a pi-section filter.

The insertion loss I_(L) of the Pi filter is given by:$I_{L} = {20\log_{10}\frac{Z_{L}\left( {{Z_{1}^{2}Z_{3}} + {Z_{1}Z_{3}^{2}} + {Z_{1}Z_{2}Z_{3}}} \right.}{P}}$

where:

P=R _(S) [Z ₁ ² Z ₃+2Z ₁ Z ₂ Z ₃ +Z ₂ ² Z ₃ +Z ₁ Z ₃ ² +

Z₂ Z ₃ ² ]

+R_(S) Z ^(L) [Z ₁ ₂+2Z ₁ Z ₂+2Z ¹ Z ³ +Z ₂ ²+2Z ₂ Z ₃ +

Z₃ ² ]

+Z_(L) [Z ₁ ² Z ₂ +Z ₁ Z ₂ ²+2Z ₁ Z ₂ Z ₃ +Z ₁ ² Z ₃ +Z ₁

Z₃ ² ]

+Z₁ ² Z ₂ Z ₃ +Z ₁ Z ₂ ² Z ₃ +Z ₁ Z ₂ Z ₃ ²  [Equ (20)]

and where: $Z_{1} = \frac{1}{{j\omega}\quad C_{1}}$

Z₂=R_(e)+jωL_(e)$Z_{3} = \frac{1}{{j\omega}\left( {C_{L} + C_{2}} \right)}$

Z_(L)=R_(L).

Extension to other filter characteristics

In the case of high-pass L-C filters that respectively correspond inconfiguration to the low-pass filters of FIGS. 2 to 4 after changing Lcomponents to C components and vice versa, the forms of the log-logcharacteristic curves are for many purposes substantially similar exceptfor the reversal of the log-frequency scale, i.e. in the case of thehigh-pass filters the low-frequency end is to the right of each plotinstead of to the left.

Several other types of filter show other characteristic data patternsand differences between serviceable and faulty examples, as may beascertained by the reader skilled in the art by using apparatusembodying the present invention, e.g. that of FIG. 1, to obtain data andcurves corresponding to those of FIGS. 2B, 3C and 4B in the case of eachof several filter types that may be of interest, using serviceable andfaulty examples.

In addition to the use of the test system and method as described todetect fault conditions such as short circuits and open circuits, andcomponent values, the system and method can also be used to detect othercharacteristics of filters or circuit elements under test. For example,also detectable are changes, e.g. by way of deterioration, with time,and/or with use or wear, of the component values in such filters orcircuit elements, or changes in value of the load impedances connectedto such elements.

Where alternative measurement circuit arrangements are used, e.g.current measurement or use of a high-impedance constant-current source,corresponding changes generally need to be made in handling the forms ofdata that result, as will be appreciated by the skilled reader.

The present invention is susceptible to many other modifications andvariations and the present disclosure extends to other combinations andsubcombinations of the features mentioned above and illustrated in thedrawings.

TABLE 1 Test Frequency Test voltage ratio 10 kHz (datum frequency)V_(filter) f_(cut-off) V_(filter) − 3 dB f_(cut-off) × 3.162 V_(filter)− 10 dB f_(cut-off) × 10 V_(filter) − 20 dB f_(cut-off) × 31.62V_(filter) − 30 dB f_(cut-off) × 100 V_(filter) − 40 dB f_(cut-off) ×316.2 V_(filter) − 50 dB f_(cut-off) × 1000 V_(filter) − 60 dB

What is claimed is:
 1. A method of testing a frequency-dependentelectrical circuit with input and output connections and of knowncomponent configuration, the method comprising the steps of: (a)providing an electrical circuit to be tested, the circuit having inputand output connections; (b) applying a.c. input test signalssuccessively over a range of frequencies to the input connections of thecircuit to be tested; (c) measuring, at each frequency, the magnitude ofthe test signal across the input connections of the electrical circuitbeing tested; (d) determining, from the magnitudes of the test signalsat each of the range of frequencies, a tested frequency response of thecircuit and identifying the shape of the characteristic frequencyresponse for the known component configuration; (e) comparing the shapeof the characteristic frequency response to the tested frequencyresponse; (f) determining automatically, from the results of thefrequency response comparison, whether the circuit is operational orfaulty; (g) determining, from the magnitudes of the test signals at eachof the range of frequencies, component values for the electricalcircuit; and (h) determining the insertion loss of the electricalcircuit from the component values.
 2. The method according to claim 1,wherein the magnitude of the test signal is measured by taking the RMSvoltage at the input connections.
 3. The method according to claim 1,wherein the method of testing the circuit is fully automated using acomputer.
 4. The method according to claim 1, wherein the circuit to betested comprises a high-pass filter.
 5. The method according to claim 1,wherein the circuit to be tested comprises a band-pass filter.
 6. Themethod of claim 1, wherein the circuit to be tested comprises a filter.7. The method of claim 1, including the step of providing a load acrossthe output connections of the electrical circuit to be tested.
 8. Amethod of testing a frequency-dependent electrical circuit with inputand output connections and of known component configuration, the methodcomprising the steps of: (a) providing an electrical circuit to betested, the circuit having input and output connections; (b) applyinga.c. input test signals successively over a range of frequencies to theinput connections of the circuit to be tested; (c) measuring, at eachfrequency, the magnitude of the test signal across the input connectionsof the electrical circuit being tested; (d) determining, from themagnitudes of the test signals at each of the range of frequencies, atested frequency response of the circuit and identifying the shape ofthe characteristic frequency response for the known componentconfiguration; (e) comparing the shape of the characteristic frequencyresponse to the tested frequency response; (f) determiningautomatically, from the results of the frequency response comparison,whether the circuit is operational or faulty; (g) determining, from themagnitudes of the test signals at each of the range of frequencies,component values for the electrical circuit; and (h) determining theinsertion loss of the electrical circuit from the component values,wherein the circuit to be tested is a C-section filter.
 9. The methodaccording to claim 8, wherein the magnitude of the test signal ismeasured by taking the RMS voltage at the input connections.
 10. Amethod of testing a frequency-dependent electrical circuit with inputand output connections and of known component configuration, the methodcomprising the steps of: (a) providing an electrical circuit to betested, the circuit having input and output connections; (b) applyinga.c. input test signals successively over a range of frequencies to theinput connections of the circuit to be tested; (c) measuring, at eachfrequency, the magnitude of the test signal across the input connectionsof the electrical circuit being tested; (d) determining, from themagnitudes of the test signals at each of the range of frequencies, atested frequency response of the circuit and identifying the shape ofthe characteristic frequency response for the known componentconfiguration; (e) comparing the shape of the characteristic frequencyresponse to the tested frequency response; (f) determiningautomatically, from the results of the frequency response comparison,whether the circuit is operational or faulty; (g) determining, from themagnitudes of the test signals at each of the range of frequencies,component values for the electrical circuit; and (h) determining theinsertion loss of the electrical circuit from the component values,wherein the circuit to be tested is an L-section filter.
 11. The methodaccording to claim 10, wherein the magnitude of the test signal ismeasured by taking the RMS voltage at the input connections.
 12. Amethod of testing a frequency-dependent electrical circuit with inputand output connections and of known component configuration, the methodcomprising the steps of: (a) providing an electrical circuit to betested, the circuit having input and output connections; (b) applyinga.c. input test signals successively over a range of frequencies to theinput connections of the circuit to be tested; (c) measuring, at eachfrequency, the magnitude of the test signal across the input connectionsof the electrical circuit being tested; (d) determining, from themagnitudes of the test signals at each of the range of frequencies, atested frequency response of the circuit and identifying the shape ofthe characteristic frequency response for the known componentconfiguration; (e) comparing the shape of the characteristic frequencyresponse to the tested frequency response; (f) determiningautomatically, from the results of the frequency response comparison,whether the circuit is operational or faulty; (g) determining, from themagnitudes of the test signals at each of the range of frequencies,component values for the electrical circuit; and (h) determining theinsertion loss of the electrical circuit from the component values,wherein the circuit to be tested-is a T-section filter.
 13. The methodaccording to claim 12, wherein the magnitude of the test signal ismeasured by taking the RMS voltage at the input connections.
 14. Amethod of testing a frequency-dependent electrical circuit with inputand output connections and of known component configuration, the methodcomprising the steps of: (a) providing an electrical circuit to betested, the circuit having input and output connections; (b) applyinga.c. input test signals successively over a range of frequencies to theinput connections of the circuit to be tested; (c) measuring, at eachfrequency, the magnitude of the test signal across the input connectionsof the electrical circuit being tested; (d) determining, from themagnitudes of the test signals at each of the range of frequencies, atested frequency response of the circuit and identifying the shape ofthe characteristic frequency response for the known componentconfiguration; (e) comparing the shape of the characteristic frequencyresponse to the tested frequency response; (f) determiningautomatically, from the results of the frequency response comparison,whether the circuit is operational or faulty; (g) determining, from themagnitudes of the test signals at each of the range of frequencies,component values for the electrical circuit; and (h) determining theinsertion loss of the electrical circuit from the component values,wherein the circuit to be tested is a pi-section filter.
 15. The methodaccording to claim 14, wherein the magnitude of the test signal ismeasured by taking the RMS voltage at the input connections. 16.Apparatus for testing a frequency-dependent electrical circuit withinput and output connections and of known component configuration, theapparatus comprising: (a) a signal source for applying a.c. input testsignals successively over a range of frequencies to the inputconnections of the electrical circuit to be tested; (b) a meter formeasuring, at each frequency, the magnitude of the test signal acrossthe input connections of the electrical circuit being tested; (c) meansfor determining, from the magnitudes of the test signals at each of therange of frequencies, a tested frequency response of the circuit andidentifying the shape of the characteristic frequency response for theknown component configuration; (d) means for comparing the shape of thecharacteristic frequency response to the tested frequency response; (e)means for determining automatically, from the results of the frequencyresponse comparison, whether the circuit is operational or faulty; (f)means for determining, from the magnitudes of the test signals at eachof the range of frequencies, the component values for the electricalcircuit; and (g) means for determining insertion loss of the circuitfrom the component values.
 17. The apparatus according to claim 16,wherein the meter measures the magnitude of the test signal by takingthe RMS voltage at the input connections.
 18. The apparatus according toclaim 16, including a computer comprising said means for determining andidentifying means for comparing, said means for determiningautomatically, said means for determining component values, and saidmeans for determining insertion loss.
 19. The apparatus of claim 16,including a load connected across the output connections of thefrequency-dependent electrical circuit.
 20. The apparatus according toclaim 16, wherein the circuit to be tested comprises a filter.